The present invention relates to an amplifier, for amplifying an analog signal or a digital signal, an output stage of which is a bridge structure.
In the related art, for example, in a class-D amplifier used as an amplifier for driving a loudspeaker, there is known a class D amplifier that inputs an analog or digital audio signal to perform PWM (Pulse Width Modulation) on the signal and amplifies the PWM-modulated signal to drive a load circuit with a bridge structure driver (by using a BTL or Bridged Transformer Less connection).
FIG. 7 is a block diagram of a related art class D amplifier including a PWM modulator 71 two pre-drivers 72, 73 and a driver 74. The driver 74 includes-four switches. In FIG. 7, the driver 74 is shown as two drivers 741 and two drivers 742. The PWM modulator 71 input an analog or digital audio signal to perform PWM modulation on the signal and outputs the modulated signal to the pre-drivers 72, 73. The PWM modulator 71 outputs a signal +IN for output to a +OUT terminal to the pre-driver 72 and outputs a signal −IN for output to an −OUT terminal to the pre-driver 73.
In the related art class D amplifier, a modulation method is used that reduces the output at the time of the absence of a signal to substantially zero in order to reduce power losses (for example, refer to Patent Reference 1). The relationship between input and output in this method is shown in FIG. 8. While input is represented as an analog value in FIG. 8, a digital value as input results in the same operation.
In FIG. 8, when an input signal is positive, a PWM-modulated signal is output to the +OUT terminal. When an input signal is negative, a PWM-modulated signal is output to the −OUT terminal. Because the load circuit is connected between the +OUT terminal and the −OUT terminal, the difference (+OUT)−(−OUT) between the output signal from the +OUT terminal and the output signal from the −OUT terminal serves as input to the load circuit.
The pre-driver 72 inputs one PWM signal from the PWM modulator 71 and outputs a gate signal for actuating a driver 741 to the driver 741. The driver 741 amplifies a gate signal input from the pre-driver 72 and outputs a BTL signal with positive polarity from the +OUT terminal.
Similarly, the pre-driver 73 inputs the other PWM signal from the PWM modulator 72 and outputs a gate signal for actuating a driver 742 to the driver 742. The driver 742 amplifies the gate signal input from the pre-driver 73 and outputs a BTL signal with negative polarity from the −OUT terminal.
In the BTL-connection class D amplifier, when a high potential (High level) is continuously output from either the +OUT terminal or −OUT terminal, the other output is at a low potential (Low level). On that occasion, one of the two pre-drivers 72, 73 is operating and the other pre-driver is in a sleep state to output a Low level.
The switching amplifier disclosed in Patent Reference 2 simultaneously drives four switches constituting an output stage, thus requiring two separate amplifiers for driving two outputs.    Patent Reference 1: JP-A-2006-042296    Patent Reference 2: JP-A-06-152268
In the related art amplifier, one of the two pre-drivers may be in the sleep state. Two pre-drivers are required although one pre-driver circuit in the sleep state is unnecessary. This results in a larger chip area of an amplifier.